The present invention relates to medical stimulators and leads generally, and more particularly to implantable pacemakers, cardioverters and defibrillators.
In the context of implantable pacemakers or other stimulators which stimulate and sense electrical activity in multiple chambers of the heart, it has been conventional to provide a blanking period for the amplifier associated with one chamber of the heart, during delivery of a pacing pulse to another chamber of the heart. An earlier example of this feature may be found in U.S. Pat. No. 4,312,355 issued to Funke. It is also conventional to provide a blanking period for the sense amplifier coupled to the chamber being paced, during delivery of the pacing pulse and to provide atrial refractory and/or blanking periods associated with sensed ventricular depolarizations, as in U.S. Pat. No. 5,027,815, issued to Funke and U.S. Pat. No. 5,123,412, issued to Betzold.
Particularly in the context of devices which detect tachyarrhythmias, amplifiers have been developed which automatically adjust the effective sensing threshold, in order to facilitate sensing of the relatively lower amplitude depolarization wave forms that may be associated with tachyarrhythmias without sensing the repolarization wave forms associated with depolarizations occurring during normal sinus rhythm. The adjusting of the effective sensing threshold may be accomplished by adjusting the gain of the amplifier and comparing the amplified signal to a fixed threshold and/or by adjusting the threshold level of the detector associated with the amplifier, which adjustments should be understood to be equivalent alternatives in the context of the present invention. One such auto-adjusting amplifier is disclosed in U.S. Pat. No. 5,117,824 issued to Keimel et al, incorporated herein by reference in its entirety. An alternative implementation of an auto adjust amplifier is disclosed in U.S. Pat. No. 5,269,300 issued to Kelly et al., also incorporated herein by reference in its entirety. In these references, following a detected depolarization, the amplifier is automatically adjusted so that the effective sensing threshold is set to be equal to a predetermined portion of the amplitude of the sensed depolarization, and the effective sensing threshold decays thereafter to a lower or base sensing threshold. Following delivery of a pacing pulse, in the system disclosed in the Keimel et al patent, no adjustment is made to the sensing threshold, while in the Kelly et al. patent, following delivery of a pacing pulse the effective sensing threshold is set to a preset value and remains at this value for a defined period of time, after which the threshold decays to the lower or base value.
In the context of a device which paces and senses in multiple chambers of the heart, employing blanking and refractory periods as described above, alone or in conjunction with auto adjusting amplifiers as described above, does provide a useful and workable device. However, this approach does not address the difficulties which arise when the signal associated with a depolarization in the ventricle is of sufficient amplitude to be sensed by the atrial sense amplifier, commonly referred to as far-field R-wave sensing. This problem is addressed to some extent by provision of atrial blanking or refractory periods following sensing in the ventricle, but at the cost of the ability to accurately respond to atrial depolarizations occurring within these periods. In addition, the far-field R-wave may sometimes be sensed in the atrium before the R-wave is sensed by the ventricular sense amplifier, prior to initiation of blanking or refractory periods associated with the R-wave.